1. Field of the Invention
This invention relates to an under thread detection device for a sewing machine which is capable of detecting a residual amount of an under thread wound on a bobbin without taking the bobbin out of a bobbin housing.
2. Description of the Prior Art
Upon sewing apparel cloths with the use of a sewing machine, there is the possibility that a sewing operator may unwittingly exhaust the under thread from a bobbin while doing the sewing operation. Once the under thread is exhausted from the bobbin while doing the sewing operation, it leads to deterioration in the commercial value of the clothes. In order to avoid the deterioration of commercial value, it is necessary to undo the sewn portion of the apparel cloths and, depending the clothes (e.g., suit) to be sewn again sew the clothes correctly.
There are various ways to recognize the residual amount of the under thread. One is to presuppose it, from the sewing amount required to finish the apparel, when exchanging bobbins. Another way is to visually recognize the residual amount of the under thread by taking out the bobbin before restarting the sewing operation.
However, the former has a drawback in that the under thread may be unwittingly exhausted while doing the sewing operation, particularly when an amount of the under thread wound on the bobbin is small. The latter also has a disadvantage in that sewing workability is reduced because it is necessary to take the bobbin out of the bobbin housing each time the sewing operation is started.
With the above situation as a backgound, although it has been demanded that an under thread detection device be introduced in the market place, it is very difficult to install the under thread detection device on a limited space around a rotary bobbin frame to which the bobbin is placed.
Even if the the under thread detection device can be installed around the rotary bobbin frame of a specified type sewing machine, it can not always be installed on other types of sewing machines because a space around a rotary bobbin frame is different depending on the type of a sewing machine. Thus it is difficult to mass produce the under thread detection device, rendering the device costly.
Even when installing the device on the limited space around the rotary bobbin frame, the device is likely to undergo malfunction because the device is in such an adverse environment as to subject it to a great amount of wasted thread and lubricating oil jumping across the space in combination with the rotary movement of the bobbin frame.
Therefore, it is a main object of the invention to provide an under thread detection device suited for a general purpose application which is capable of recognizing a residual amount of an under thread without taking the bobbin outside the bobbin frame while eliminating the malfunction due to the wasted thread and lubricating oil.